First of all, make sure they have a clean IE cache, restart their browser, etc.
Make sure they donβt have the wrong username and password for the hostname of the site in your Windows profile.
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stsadm -o getsiteuseraccountdirectorypath -url <site url>
stsadm -o getproperty -pn peoplepicker-seachadcustomfilter -url <site url>